chore: change form boundary length to 50 characters

[keyurgovrani]

Reduced the boundary length for multipart form generation from 70 to 50.

Summary:

This change reduces the automatically generated multipart form-data boundary length from 70 characters to 50 characters in RCTNetworking.mm.

The motivation for this change is to prevent request failures caused by backend constraints on the Content-Type header length.

The Content-Type header is formatted as:

multipart/form-data; boundary=<BOUNDARY_VALUE>

Our backend enforces a maximum header length limit of 90 characters. With a 70-character boundary, the total Content-Type header length exceeded this limit (crossing 100 characters), leading to request rejection.

By reducing the boundary length to 50 characters, the total header length now remains within the allowed limit while preserving correct multipart behavior.

Changelog:

[GENERAL] [FIXED] - Reduced multipart form-data boundary length to prevent oversized Content-Type header failures

Test Plan:

• Verified multipart/form-data requests succeed against backend with 90-character Content-Type header limit.
• Confirmed total header length is now within allowed constraints.
• Tested file uploads to ensure multipart request body integrity remains intact.
• No regressions observed in upload functionality.

[meta-cla]

Hi @keyurgovrani!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[cipolleschi]

We can't really approve this request: you are about to change the limit for EVERY React native application because of your specific server:

Our backend enforces a maximum header length limit of 90 characters.

This is not acceptable. A reasonable fix would be to make this configurable from the app, leaving the base value of 70 chars. You will be then able to customize React Native for your specific use case, without changing it for all the users.

[nil1511]

Thanks for the context! @cipolleschi That's a valid concern about changing a global default.

That said, it's worth noting that React Native iOS is actually an outlier here — all major platforms use significantly shorter boundaries: Chromium and Safari use 16 characters, Firefox uses 32, and React Native Android via OkHttp uses 36 (source). A 70-character boundary on iOS produces headers that exceed limits enforced by several enterprise security policies.

Would the project be open to aligning iOS with Android by adopting a 36-character boundary to match OkHttp? This would improve cross-platform consistency within React Native itself, follow established precedent from widely adopted runtimes, and broadly benefit users without requiring per-app security configuration for enterprises.